The invention relates to a controller for a continuously variable speed transmission comprising a hydraulic pump and a hydraulic motor.
With a continuously variable speed transmission comprising a closed circuit disposed between a hydraulic pump and a hydraulic motor at least one of which is of variable displacement type, as disclosed in the Japanese Patent laid-open publication No. 95722/1981, a bypass line is provided between the pump and motor to communicate two hydraulic lines constituting the closed circuit, and the continuously variable speed transmission is clutch-controlled by opening and closing the bypass line with an opening control valve (clutch valve).
When a vehicle with such a continuously variable speed transmission mounted thereon is decelerated by releasing the depression of the acceleration pedal, such control has been adopted as to prevent excessive engine brake and to maintain smooth running by opening the clutch valve when the vehicle speed falls below a given value see, e.g., to Japanese Patent laid-open publication No. 127562/1987).
Such control, however, when such aggressive braking is applied that wheel locking occurs, may cause engine stall due to overloading caused by the wheel locking before the vehicle speed comes down to the given value. To avoid this problem, a proposal has been made to open the clutch valve as soon as the service brakes are applied (see Japanese laid-open publication No. 207229/1986). But, in this case, even a light depressing of the brake pedal may release the clutch valve, therefore making engine braking not available.